Vacuum sewage systems, particularly vacuum toilet systems, are well known. At a first stage of development a vacuum sewage system typically comprised a large volume collecting tank, in which system the tank is maintained under vacuum in order to provide vacuum for the vacuum sewer piping. Vacuum is maintained in the large volume collecting tank by a separate vacuum pump. The sewage is emptied from the collecting tank by a sewage pump connected to the bottom of the collecting tank. Due to the large volume collecting tank, such systems require a lot of space, which normally is not available in e.g. trains, marine vessels and aircraft. An example of such a solution is disclosed e.g. in EP 0 330 490.
At a later stage, an ejector based system came along, in which sewage is circulated from a large volume collecting tank to a liquid jet pump opening into the same collecting tank. The liquid jet pump has an inlet connected to vacuum sewer piping and further to sanitary units, such as toilets. The sewage is circulated through the liquid jet pump, whereby the liquid jet pump generates a vacuum towards the vacuum sewer piping for drawing sewage from the toilets into the collecting tank. This solution requires a lot of space. Further, it has a low degree of efficiency. Such a solution is known e.g. from EP 0 653 524.
Other developments include a liquid ring pump that is connected to a large volume collecting tank, whereby vacuum is generated and sewage is pumped by the same pump in an alternating manner, e.g. EP 0 287 350.
Another single pump solution is disclosed in EP 0 644 299. This solution comprises a moisture protected dry rotary vane pump, which generates vacuum in a large volume collecting tank. This tank is emptied by reversing the pump function, so that the pressure side of the pump is connected to the large volume collecting tank for emptying the same by forcing out the sewage to another location by pressurized air. This solution also requires a large space and furthermore the pump is very vulnerable to any humidity, thus requiring intricate control mechanisms.
On-line systems provided with liquid ring pumps generating vacuum and pumping sewage directly from the vacuum sewer piping are also known and are disclosed e.g. in EP 0 333 045 and EP 0 454 794. A further on-line solution is described e.g. in EP 1 172 492. Such on-line systems deploy one single pump which generates vacuum and sucks sewage at the same time. These solutions represent attempts to reduce space requirement by eliminating the intermediate large volume collecting tank, i.e. a tank located between the pump and the vacuum sewer piping,
The latter solutions with on-line pumps having the dual duty of generating vacuum by sucking air and simultaneously sucking sewage are, however, not very efficient. Further, they are apt to functional disturbances. The pumps used in this context are designed either for pumping air or pumping liquid, whereby the transport of both air and liquid generally is not very successful.
This is due to the typical transport function of a vacuum sewage system, particularly a vacuum toilet system, where sewage is transported through the vacuum sewer piping in slugs with intermediate large volumes of air forming a non-homogenous sewage flow.
When a vacuum toilet is flushed by activating the flush function, the discharge valve between the vacuum toilet bowl and the vacuum sewer piping is opened, and the vacuum prevailing in the vacuum sewer piping draws out the sewage and flush water from the toilet into the vacuum sewer piping. Only a small amount of flush water is needed, due to the strong suction effect of the vacuum sewer piping and the atmospheric pressure prevailing in (and around) the vacuum toilet bowl. The amount of sewage and flush water is typically about 2 liters.
Consequently, there is a pressure difference, i.e. atmospheric pressure on the toilet bowl side of the sewage and flush water and vacuum on the vacuum sewer piping side of the sewage and flush water, when the discharge valve opens. The transport of sewage and flush water takes place due to this pressure difference, whereby the sewage and flush water forms a discrete slug followed by a large amount of air, e.g. about 2 liters of sewage and flush water followed by about 60 liters of air, i.e. a sewage and flush water in a ratio of about 1:30 to air. A large amount of air is sucked or forced into the vacuum sewer piping since the discharge valve remains open for a certain period of time.
Vacuum sewer piping generally is of a relatively small diameter which helps to keep up the formation of the slugs, which again is necessary for maintaining the pressure difference (lower pressure in front and higher pressure behind the discrete slugs) necessary for transport. During the transport through the vacuum sewer piping, the slugs are affected by gravity and flatten out after some time in horizontally arranged piping. This equalizes the pressure, whereby the necessary pressure difference described above before and after the discrete slugs is abolished. In order to re-establish the pressure difference for sewage transport, the vacuum sewer piping is provided with low points or pockets, in which the sewage collects so that the discrete slugs are formed again.
Such a train of discrete slugs and air, especially in a ratio of about 1:30, which forms the sewage flow in a vacuum sewage system, is difficult to pump with high efficiency.